1. Field of the Invention
The present invention relates to a method for producing expandable thermoplastic polymer particles at a high yield, particularly expandable styrene type resin particles, giving cells formed inside the expanded particles which are fine and uniform in size even when expansion is carried out immediately after production of the particles.
2. Description of the Prior Art
Expandable thermoplastic polymer particles release an easily volatile blowing agent (hereinafter referred to as "blowing agent") contained therein to thereby lose its expanding capability readily, and the time required therefor becomes shorter with particles smaller in size. When a mixture of particles different in size is expanded, a difference in expansion ratio results according to the particle size and the strength of foamed articles identical in density is lowered with increasing proportion of large-sized particles. When large-sized pre-expanded particles are included, it gives rise to such problems as deterioration of the filling property into a mold and difficulty to obtain a good foamed article. Furthermore, a foamed article obtained from particles broad in particle size distribution is different in the size of expanded particles, hence not favorable in external aesthetic beauty. From such a viewpoint, it is preferred to take those narrow in particle size as expandable thermoplastic polymer particles for producing a foamed article.
For obtaining expandable thermoplastic polymer particles, there have hitherto been known methods roughly divided into the following categories;
(1) A method for obtaining particles of a given size by suspension polymerization of a polymerizable monomer, then impregnating the resulting polymer with a blowing agent and finally sifting it.
(2) method for suspension polymerizing a polymerizable monomer, then sifting the resulting polymer and finally impregnating only the particles of a given range of size with a blowing agent.
(3) A method for pelletizing a polymer and then impregnating the resulting pellets of a given size with a blowing agent.
(4) The so-called seed polymerization method in which a polymerizable monomer is suspension polymerized, then only particles of a given size are taken out and suspended in water, subsequently a polymerizable monomer is added either continuously or intermittently for further polymerization, then the grown particles are impregnated with a blowing agent.
As to the method (1), however, all particles are impregnated with the blowing agent, and hence, the particles containing the blowing agent but not suited for intended uses, because of their particle size being outside the intended range have to be treated for some other use, which becomes a heavier burden with increasing scale of production.
As to the method (2), since the polymer particles are used after sifting, the process of producing polymer particles and that of impregnating them with the blowing agent are necessarily separated, which results in complication of the process and also increased cost. Further, by this method, too, like the method (1), the burden of treating the particles off the desired range of the particle size for some other suitable use cannot be avoided, either.
In the method (3), too, the pelletizing step is required in addition to the polymer producing step and the step of impregnation with the blowing agent, the overall process being thus highly complicated. The cells formed by expanding the expandable particles are extremely small and it is difficult to obtain a good foamed article.
In order to eliminate the aforementioned defects, the method (4) disclosed in Japanese Patent Publication No. 2994/1974, the so-called seed polymerization method, is proposed. By this method, production of expandable thermoplastic polymer particles substantially in the desired particle size is feasible. Meanwhile, in any of the methods (1)-(4), the number of cells formed inside the individual particles is small when the obtained particles are expanded immediately after production to produce pre-expanded particles and, moreover, such cells are bound to be coarse and large. In order to make these cells uniform and fine, it was a usual practice to provide an aging period of 1-3 weeks but this method had a drawback of the blowing agent being lost in the aging period, thus resulting in lowering of expanding capability. As to the producing methods (1), (2) and the like, the possibility of shortening or omitting the aging period by conducting polymerization with addition of various amide compounds simultaneously with a monomer is disclosed in Japanese Laid-open Publication Nos. 48588/1973, 161441/1984, 161442/1984, 166538/1984 and 168037/1984.
Notwithstanding, by the (4) seed polymerization method in which polymerization reaction proceeds at a high conversion ratio, it has been considered difficult to shorten the aging period by the use of any amide compound since particles formed at such a high conversion ratio are not easily impregnated with any amide compound to the interior thereof, hence such possibility has not been studied yet.